The present invention relates to draft inducers for use with fossil fuel burning hot water heaters and also to other devices, such as furnaces and HVAC systems that may require a draft inducer. The following application will be described with respect to hot water heaters, but should not be seen as being limited as such to hot water heaters. Hot water heaters are only provided as an example.
Hot water heaters take in cold water and heat the water in a tank by burning a fuel such as gas or oil. The burning of the fuel produces an exhaust. In typical hot water heaters the tank is held inside of an outer structure so as to allow the heat to be transferred to the inner tank and to the water. The fumes that are produced by the burning of the fuel are extremely hot and need to be vented away from the outer structure of the hot water heater in order to prevent pressure buildup on the outer structure and the possibility that the structure will explode. The gasses rise up to the top of the outer structure and need to be safely exhausted.
In prior art systems, the hot gas produced by a hot water heater is coupled directly to metal piping for venting the hot gas to an outside location. Metal piping is rather expensive and draft inducers were developed in order to reduce the temperature of the hot gas, so that cheaper piping, such as PVC tubing could be used in place of the metal piping. Further, PVC piping allows routing of the exhaust in non-linear directions which cannot be performed with metal piping.
In a typical hot water heater, the hot gases produced by burning the fuel to heat the water are vented between the hot water outlet pipe and the cold water inlet pipes that are found on the top of a hot water heater. Therefore, draft inducers are typically mounted between the cold and hot water pipes on the top of a hot water heater. The distance between the two pipes is on the order of seven inches in a typical hot water heater. Because of the typical line frequency in the power lines of 50-60 Hz, AC motors cannot rotate above 3200-3400 RPM. As a result of the speed limitation of AC motors, in order to provide proper ventilation, the size of the prior art forward-curved blades of the impeller must be so large that the draft inducer cannot be mounted horizontally between the hot and cold water pipes of a hot water heater. Therefore, the draft inducer would necessarily have to be mounted in a vertical configuration, such that the impeller would spin about a horizontal axis with respect to gravity.
The distance between the hot and cold water pipes is not large enough to accommodate a draft inducer having a horizontally spinning impeller with forward curved blades and coupled to an AC motor. In addition to being aesthetically unpleasant, such large prior art draft inducers produce excessive noise.
AC motors and forward curved impeller blades were used by prior art systems because AC motors exhibit a flat torque/speed curve. As such, even if the torque drops, the speed of the motor remains nearly constant. This characteristic was viewed as desirable, since the impedance on the draft inducer from application to application varies. For example, the length of the exhaust pipe that the draft inducer is used to drive may vary from approximately 1 foot to lengths of 45 feet or more. As such, the draft inducer with an AC motor would be able to drive the gases through the exhaust piping regardless of the length of the exhaust pipe. However, this causes the hot water heater to operate inefficiently if the exhaust pipe has a short length, since more gas is pulled out of the hot water heater along with the accompanying heat. Thus there is less energy present to heat the hot water and the hot water heater must use additional energy to obtain the desired water temperature.